1. Field of the Invention
The present invention relates to a mold for producing a silicon ingot for use in producing a polycrystalline silicon ingot to be used for a silicon substrate for a photovoltaic solar cell, and to a method for fabricating the same. The present invention especially relates to a mold for producing a silicon ingot having an inner silica layer, or an inner silica layer and outer silica layer, being free from crack due to inner stress caused by cooling after solidification of the molten silicon, and a method for fabricating the same.
2. Description of the Related Art
A quartz mold or a graphite mold has been used as a mold for producing a silicon ingot. The quartz mold is fabricated by the steps comprising: preparing fused silica powder, filling the fused silica powder into a cavity of a graphite mold comprising an inner mold and outer mold having a cavity with the same shape as the mold; rotating the outer mold while firing a burner flame on the fused silica powder adhered on inner side wall of the outer mold after removing the inner mold; and forming the silica powder into a mold shape while the powder is melting. The cross section of the conventional quartz mold fabricated as described above is shown in FIG. 9. Otherwise, the graphite mold is fabricated by assembling graphite plates. Because detachability of the ingot from the mold is so poor, sometimes causing cracks in the ingot, when the mold is directly used after fabrication that the surface of the mold should be treated with an inert coating substance. Accordingly, the inside wall of the mold is coated with a slurry prepared by mixing a powder of silicon dioxide (SiO2), silicon carbide (SiC), silicon nitride (Si3N4), or yttrium oxide (Y2O3) with 4% aqueous polyvinyl alcohol solution as a release agent at a thickness of 0.5 mm or less.
However, the silicon ingot 2 is subjected to heat-shrink when the molten silicon is cooled in the quartz mold 1. Since the silicon ingot is heat-shrunk along the direction indicated by S in FIG. 9 while remaining to be adhered to the wall face of the quartz mold 1, the outer circumference of the silicon ingot 2 suffers a tension at the inner wall of the quartz mold to generate dislocations or cracks due to inner stress inside of the ingot 2. The silicon substrate for use in a photovoltaic solar cell, produced from the silicon ingot having cracks or dislocations, inevitably has poor photovoltaic efficiency.
On the other hand, particles of the release agent freed from the surface of the mold penetrate into the molten silicon when the molten silicon is allowed to stand for a long period of time in the mold coated with the release agent. Consequently, a silicon substrate with excellent photovoltaic conversion efficiency can not be obtained because particles of the release agent are enveloped into the silicon ingot produced using the mold coated with the release agent.
Meanwhile, producing a high purity silicon ingot with a more reasonable production cost is crucial cost down requirements for silicon substrate production.
Accordingly, the object of the present invention is to provide a silicon ingot with fewer cracks, dislocations and no release agent with reasonable cost than the ingot produced by using the conventional quartz mold.
In one aspect, the present invention provides a mold for producing a silicon ingot having a layered structure comprising an inner silica layer containing at least one layer in which a fused silica powder with a particle size of 100 xcexcm or less and fine fused silica sand with a particle size of 100-300 xcexcm is joined with a silica binder, and an outer silica layer containing at least one layer in which a fused silica powder with a particle size of 100 xcexcm or less and coarse fused silica sand with a particle size of 500-1500 xcexcm is bonded with a silica binder.
In accordance with another aspect, the present invention provides a method for fabricating a mold for producing a silicon ingot having an inner silica layer and an outer silica layer comprising the steps of:
forming a slurry layer on the surface of a wax mold by dipping, followed by lifting up, the wax mold having the same shape as that of the inner shape of the mold for producing a silicon ingot into a slurry comprising fused silica powder and colloidal silica, followed by forming an inner stucco layer by sprinkling fine fused silica sand with a particle size of 100-300 xcexcm on the surface of the slurry layer;
forming an outer stucco layer on the inner stucco layer by sprinkling coarse fused silica sand with a particle size of 500-1500 xcexcm on the surface of the slurry layer after forming the slurry layer by further dipping, followed by lifting up, the wax mold on which the inner stucco layer has been formed into a slurry comprising the fused silica powder and colloidal silica; and
heat-melting and eliminating the wax mold along with baking the inner stucco layer and outer stucco layer.